Matching Materials To Bandpass Filters

Bandpass Filters, Part 2

Bandpass filters make many of our modern electronic systems possible, from dependable cellular telephones to secretive military surveillance and radar systems. The last blog—Part 1 of this two-part series on bandpass filters—highlighted the versatility of one circuit material from Rogers Corporation, RT/duroid® 6010.2LM laminate, for fabricating RF/microwave bandpass filters. But not all circuit materials are the same and there may be some advantages to designing bandpass filters on other materials, such as Rogers RO4000® family of printed-circuit-board (PCB) materials. This blog will examine different grades of these and other circuit materials and the impact they have on the design and fabrication of high-frequency bandpass filters, especially compared to filters formed on filled-PTFE-based circuit materials.

The first installment of this two-part blog on bandpass filters described different bandpass filter responses, including Chebyshev, Bessel, and Butterworth filters, and reviewed a number of the performance parameters for comparing bandpass filters, such as center frequency, passband, passband insertion loss, and stopband responses. As was noted in that first part, the choice of PCB material for a bandpass filter usually starts with the material’s dielectric constant (Dk). Materials with higher values of Dk, such as 10.2, have long been favored for RF/microwave filters because of the relationship of the Dk value to the size of the filter. Quite simply, higher Dk values result in shorter wavelengths and higher frequencies, enabling filter designers to occupy less PCB area for a given filter structure.

Circuit materials with a Dk of 10.2 are typically based on polytetrafluoroethylene (PTFE), which provides excellent electrical characteristics but tends to be more expensive than other circuit materials. As pointed out in the first part of this blog, circuit materials based on PTFE with some form of filler are also susceptible to moisture absorption, which can result in shifts in their Dk values in high-humidity environments. As that earlier blog suggested, a lower-cost material such as RT/duroid 6010.2LM laminate, which is a composite of PTFE and ceramic materials, can also provide the high Dk value without the concern for moisture absorption.

But are there benefits to designing and fabricating bandpass filters on materials with lower values of Dk? Rogers RO4000 circuit materials are widely favored by amplifier designers for their mechanical and electrical stability, but they are also excellent starting points for bandpass filters. The materials are based on reinforced hydrocarbon/ceramic laminates, not PTFE. RO4360™ laminates, for example, have a Dk value of 6.15 in the z-axis at 10 GHz, held to an impressive tolerance of ±0.15. The material is based on a ceramic-filled, thermoset-resin system reinforced by glass fiber for excellent mechanical stability. These lead-free-process-capable laminates exhibit dissipation factor (loss) of 0.0030 at 2.5 GHz and 0.0038 at 10 GHz, both in the material’s z-axis.

In terms of bandpass filter size, the lower Dk value of 6.15 for RO4360 laminates compared to a Dk of 10 or higher for filled PTFE circuit materials translates into somewhat larger filter structures, with wider conductor widths. The higher loss of RO4360 laminates compared to filled-PTFE-based materials typically means somewhat higher passband insertion loss, but the differences in loss between filters formed on the two materials may not be so significant when factoring in RO4360 laminate’s wider conductors. RO4360 laminates offer lower material costs than filled-PTFE-based substrates, and circuits are easier and lower in cost to fabricate compared to filled PTFE. RO4360 laminates provide improved mechanical stability and consistency compared to filled-PTFE-based materials, especially in environments with high humidity. Essentially, the tradeoffs between the two materials involve cost versus the levels of performance required for a particular application, as well as the somewhat smaller size possible for high-frequency bandpass filters fabricated on materials with higher Dk values.

Larger filter dimensions may not always be a design goal but at times can be a benefit, especially for applications that involve higher power levels. When designing and fabricating filters on higher-Dk materials, transmission-line conductor widths must be reduced compared to lower-Dk circuit materials to maintain the typical 50-Ω impedance of higher-frequency designs. But those thinner conductor widths, along with the circuit material’s thermal properties such as its thermal conductivity, will serve as a limit for a filter’s power-handling capabilities on that particular material. Also, the thinner conductor widths can result in penalties in terms of production yields.

RO4360 circuit materials provide better thermal conductivity than many substrates based on filled PTFE, although with somewhat higher loss that can in part offset the enhanced thermal conductivity. RO4360 laminates provide typical thermal conductivity of 0.8 W/m/K, enabling it to dissipate heat produced by circuits handling high power levels. In addition, RO4360 laminates have coefficients of thermal expansion (CTE) of 16.6 and 14.6 ppm/°C, respectively, in the x and y directions, very closely matched to copper in support of good circuit reliability at higher power levels.

For some small sacrifice in passband insertion-loss performance compared to filled-PTFE-based materials, RO4360 circuit materials can deliver RF/microwave filters without complicated production processes. The thermoset material can be handled in much the same way as low-cost, epoxy-based FR-4 circuit materials, and even readily combined with these lower-cost materials as part of a multilayer circuit structure. Often, a circuit design is “segmented” by material type in a multilayer design, with higher-frequency circuits, such as RF/microwave bandpass filters, fabricated on materials such as RO4360 laminates and less-critical circuits, such as power supplies, formed on lower-cost circuit materials such as FR-4.

Compatibility in processing the different materials not only simplifies production, but also ensures reliability of the plated through holes (PTHs) used to electrically connect the different circuit layers in a multilayer circuit construction. The reliability of those PTHs in multilayer circuits with RO4360 laminates is also aided by the materials excellent CTE in the z-axis (30 PPM/ºC), also closely matched to that of copper to minimize stress of connections and circuitry across a range of operating temperatures.

In contrast to filled-PTFE-based materials, which require special processing measures, RO4360 laminates are compatible with standard PCB processing methods, as used with FR-4 materials. They boast a high glass transition temperature (Tg) of greater than +280ºC as assurance of handling high process temperatures. In addition to electrical and mechanical characteristics that make them attractive for fabricating RF/microwave bandpass filters, they are environmental friendly and RoHS compliant. For those bandpass-filter applications that can afford slightly higher passband loss and slightly larger circuit dimensions (than some filled-PTFE materials), RO4360 circuit materials offer a high-performance alternative that can cut both material and processing costs.

Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.